1. Field of the Invention
The present invention relates to an optical modulation element and, more particularly, to an optical modulation element which is most suitable for an optical display apparatus, an optical recording apparatus, a photo-coupling apparatus, an optical communication apparatus, an optical calculation apparatus, and the like, wherein a diffraction phenomenon of a diffraction grating is controlled utilizing a diffraction grating and a variable refractive index material such as a liquid crystal, and optical modulation such as light transmission or light shielding is performed.
2. Related Background Art
A conventional well-known optical modulation element is a so-called TN (twisted nematic) type liquid crystal display element which consists of a pair of polarizing plates arranged such that their polarizing directions are perpendicular to each other, and an element which is arranged between the pair of polarizing plates and consisted of a liquid crystal sealed between the substrate surfaces on opposing substrate surfaces of a pair of transparent substrates and made an orientation treatment having orthogonal each other. The orientation state of the liquid crystal is switched between a twisted state and a state perpendicular to the substrate surface, thereby modulating incident light. A display element of this type is widely used since its construction is simple and it can be easily driven However, since this display element transmits and shields light by utilizing two polarizing plates, its transmission rate during bleaching, i.e., during light transmission is poor. Therefore, this element is not a preferable optical modulation element in terms of light utilization efficiency.
A so-called guest-host liquid crystal display element in which dyes are mixed in liquid crystal molecules is known as another display element of the same type which utilizes a liquid crystal. However, because of the presence of the dyes, this display element has a transmission property during bleaching as low as about 75% at maximum.
Japanese Patent Examined Publication No. 3928/1978 U.S. Pat. No. 4,251,137, and so on, disclose a display element or a variable subtractive process filter element wherein a reflective type or transmitting type phase diffraction grating is combined with a liquid crystal. The elements disclosed in these articles have good light utilization efficiency. However, the element disclosed in Japanese Patent Examined Publication No. 3928/1978 has a mere ornamental effect and is not satisfactory as a display element for displaying letters and pictures and an optical modulation element for transmitting or shielding light beam. The variable subtractive process filter disclosed in U.S. Pat. No. 4,251,137 includes first and second apparatuses. In the first apparatus, a variable refractive index material such as a liquid crystal is filled between substrates, at least one of which has an optically isotropic diffraction grating structure. An electric field is applied to the liquid crystal to change a refractive index of the liquid crystal, thereby performing optical modulation by utilizing a diffraction effect of the diffraction grating. In the second apparatus, diffraction gratings are formed on a pair of opposing substrate surfaces such that their orientation directions are perpendicular to each other. A liquid crystal is filled between these substrates. And by controlling the orientation of the liquid crystal molecules and changing the refraction index of the filter element, the difference in refraction index between the material constituting the diffraction gratings and the liquid crystal is changed, thereby the spectral transmission rate characteristics being able to be changed. However, although the first apparatus has high light utilization efficiency and has high performance as the variable subtractive process filter, it cannot cope with modulation of incident light having arbitrary polarization components, and hence, the second apparatus is disclosed.
In the second apparatus, since the diffraction gratings are formed on the pair of opposing substrate surfaces whose orientation directions are perpendicular to each other, the second apparatus has a distance, i.e., a cell gap more than twice that of the diffraction grating of the first variable subtractive process filter, and causes the intensity of the electric field as the refractive index control means of the variable refractive index material to be decreased to 1/2 or less. Since the two diffraction gratings are superposed on each other, the orientation directions of the liquid crystal molecules at a boundary region are disordered, and an effective diffraction grating height is decreased. Since the two diffraction gratings are superposed on each other, the second apparatus requires the number of manufacturing processes more than twice that of the first apparatus, resulting in high cost.
When the above-mentioned technique is applied to an optical display apparatus for independently controlling respective portions, such that a pattern or dots are formed on a portion of the diffraction grating or individual patterns are formed by the diffraction grating, excluding a case wherein uniform, constant control is performed in an identical plane, as in the filter disclosed in U.S. Pat. No. 4,251,137, the two diffraction gratings must precisely overlap each other, and the entire apparatus and a control system for the liquid crystal tend to become complicated.